Glass topped tables, in which a protective glass sheet is fitted but not attached or bonded to an underlying surface of a decorative or structural nature, are well known. The glass surface is added on top not only because it is transparent but because it is also weather resistant, abrasion resistant, hard, and easy to clean. The same advantages can be had with glass covered decorative wall panels if the glass is firmly secured. Whether secured or not, the dangers presented in the event of glass fracture are evident.
Safety glasses, in which a glass sheet is rendered shatter-proof or at least shatter-resistant, are also well known. Some safety glasses are made by tempering processes while others are made by laminating layers of glass sheet, or glass and plastic sheet, together with interposed transparent adhesives. While these techniques increase the cost of the glass, conventional plate and float glasses are not often able to withstand the heavy loads and high shocks to which tables for commercial use are subjected. In some restaurants, for example, the practice is to stack tables while cleaning, and this is done in vigorous and abrupt fashion, without any particular regard for fragility.
Thus although there is substantial demand for glass surfaced decorative table and panel surfaces, economic and safety factors have hitherto limited their use. If such problems could be resolved, the visually attractive features of a glass surface could be attained along with the easy maintenance and freedom from wear under typical levels of use. In addition, it is usually inconvenient or impractical to transport, mount or secure glass covers for decorative flat table tops or wall panels, and it is preferable for aesthetic reasons to have a unitary structure without the appearance of attachment.
The standard test for measuring the impact resistance of glass sheet products involves dropping a steel ball of selected standard weight (11/2 lbs.) from increasingly greater heights until the glass cracks. The glass should withstand a drop of at least 4' to pass the test. Where a glass surface is adhered to an underlying substrate the impact resistance increases somewhat, but the improved impact resistance is not generally sufficient in prior art structures to protect against normal accidents, such as dropping a glass container filled with liquid on the table top. In addition, the physical properties of such products usually vary too much when they are manufactured in production quantities. There are a number of different causes, such as too much variation in adjacent layers, consequent presence of gaps, or variations between layers, making the product more subject to fracturing. In addition, differential shrinkage between substrate and glass layers can cause delamination, particularly near the edges. Even where good adhesion is initially obtained, delamination may take place after a period of use. It must be noted in addition that the adhesives must be transparent materials that adhere to glass without introducing an inordinate cost factor. For mechanical and strength reasons, and because adhesives heretofore known are expensive they cannot be used in substantial thicknesses.